With Your Description, Your Directory Structure should look something similar to this:

Main*
-- Index.php
|
Helpers*
--Helper.php

If You are going by the book with regards to PSR-4 Standards, Your Class definitions could look similar to the ones shown below:

Index.php

<?php
// FILE-NAME: Index.php.
// LOCATED INSIDE THE "Main" DIRECTORY
// WHICH IS PRESUMED TO BE AT THE ROOT OF YOUR APP. DIRECTORY
namespace Main; //<== NOTICE Main HERE AS THE NAMESPACE...
use Main\Helpers\Helper; //<== IMPORT THE Helper CLASS FOR USE HERE
class Index {
public function __construct(){
$class = new Helper();
}
}